System for reducing multipath fade of RF signals in a wireless data application

ABSTRACT

In a wireless LAN, a technique for mitigating the effects of signal fading that results from multipath propagation. The wireless LAN includes a number of portable clients that, in one embodiment, may be notebook computers. An access point, or hub, couples the clients to a wired network and/or to each other. Multipath effects are mitigated in one embodiment by continually varying the radiation pattern of a transmitting antenna system associated with the access point. In one embodiment, the radiation pattern of a receiving antenna system is continually varied. Numerous approaches are provided to continually vary the radiation pattern of the respective antenna system, including, but not limited to, switching between two or more antennas in an array, adjusting the physical positioning of a single or multiple antennas, adjusting the effective length of the antenna, and adjusting the respective gain and/or phase of each of one or more of the signals coupled to antennas in an array.

BACKGROUND OF THE INVENTION

The disclosure relates to wireless communications systems, and, moreparticularly, to a wireless LAN that includes at an access point acontinually varying antenna array as a technique for mitigating thedeleterious effects of multipath signal propagation.

1. Field of the Invention

The invention relates to wireless communications systems, and, moreparticularly, to a wireless LAN that includes at an access point acontinually varying antenna array as a technique for mitigating thedeleterious effects of multipath signal propagation.

2. Description of the Related Art

A wireless local area network (LAN) provides a flexible datacommunication system that may be implemented as an extension to, or asan alternative for, a wired LAN. Wireless LANs transmit and receive datausing radio frequency (RF) communications techniques to thereby minimizethe need for wired connections. In this manner, wireless LANs combinedata connectivity with user mobility.

Wireless LANs have gained strong popularity in a number of verticalmarkets, including the health-care, retail, manufacturing, warehousing,and academia. These and other industries have profited from theproductivity gains of using hand-held terminals and notebook computersto transmit real-time information to centralized hosts for processing.Today wireless LANs are becoming more widely recognized as ageneral-purpose connectivity alternative for use by a broad range ofbusiness customers. Observers have predicted a sixfold expansion of theworldwide wireless LAN market by yearend 2000, reaching more than $2billion in revenues. The widespread reliance on networking in businessand the meteoric growth of the Internet and online services are strongtestimonies to the benefits of shared data and shared resources.Wireless LANs enable users to access shared information without the needto establish a hard-wired connection. Network managers have the optionto create or augment networks without installing or relocating wires.Wireless LANs offer productivity, convenience, and cost advantages overtraditional wired networks. Those advantages largely derive from speed,flexibility and simplicity of installation, reduced cost of ownership,and scalability.

For a thorough discussion of wireless LAN technology, see Jim Greer,Wireless LANs: Implementing Interoperable Networks, Macmillan TechnicalPublishing (1999), hereby incorporated by reference. In general, theimplementation of wireless LANs may be based on one or more of a widerange of technologies, including:

Narrowband Technology. Narrowband wireless systems transmit and receivedata or information on a specific radio frequency or within a specificnarrow band of frequencies. Narrowband RF techniques strive to minimizethe bandwidth necessary to transmit information. Undesirable crosstalkbetween communications channels is avoided by carefully coordinatingdifferent users on different channel frequencies. From an implementationperspective, a salient drawback of narrowband technology is that, ingeneral, the end-user must obtain an FCC license for each site where thetechnology is employed.

Spread Spectrum Technology. Wireless LAN systems predominately usespread-spectrum technology, a wideband RF technique developed by themilitary for use in reliable, secure, mission-critical communicationsystems. Spread-spectrum techniques offer enhanced reliability,integrity, and security, at the expense of increased bandwidth. In otherwords, greater bandwidth is required than in the case of narrowbandtransmission. However, the tradeoff produces a signal that is, ineffect, more robust and thus easier to detect, provided that thereceiver is informed with the parameters of the spread-spectrum signalthat is transmitted. If a receiver is not tuned to the correctfrequency, a spread-spectrum signal appears as background noise. Thereare two fundamental types of spread spectrum technology: frequencyhopping and direct sequence.

Freguency-Hopping Spread Spectrum Technology. Frequency-hoppingspread-spectrum (FHSS) uses a narrowband carrier that changes frequencyin a pattern known to both transmitter and receiver. Properlysynchronized, the net effect is to maintain a single logical channel. Toan unintended receiver, FHSS appears to be short-duration impulse noise.

Direct-Sequence Spread Spectrum Technology. Direct-sequencespread-spectrum (DSSS) generates a redundant bit pattern for each databit that is transmitted. The bit pattern is called a chip, or chippingcode. The longer the chip, the greater the probability that the originaldata can be recovered and, concomitantly, the greater the amount of thebandwidth required. Even if one or more bits in the chip are dropped intransmission, statistical techniques embedded at the receiver recoverthe original data without the need for retransmission. To an unintendedreceiver, DSSS appears as low-power, wideband noise and is rejected(ignored) by most narrowband receivers.

Infrared Technology. Infrared (IR) represents a third availabletechnology, albeit little used in commercial wireless LANs. IR systemsuse very high frequencies, immediately below visible light in theelectromagnetic spectrum, to carry data. As is the case with light, IRcannot penetrate opaque objects and is, therefore, either directed(line-of-sight) or diffuse technology. Inexpensive directed systemsprovide very limited range (three feet) and typically are used forpersonal area networks, but are occasionally used in specific wirelessLAN applications. High performance directed IR is impracticable formobile users and is therefore used only to implement fixed sub-networks.Diffuse (or reflective) IR wireless LAN systems do not require aline-of-sight transmission path, but cells are limited to individualrooms.

In a typical wireless LAN configuration, a transmitter/receiver(transceiver) device, called an access point or, alternatively, a hub,connects to the wired network from a fixed location using standardcabling. At a minimum, the access point receives, buffers, and transmitsdata between the wireless LAN and the wired network infrastructure. Asingle access point can support a small group of users and can functionwithin a range of less than one hundred to several hundred feet. Theaccess point, or the antenna attached to the access point, is usuallyelevated, but may be mounted essentially anywhere that is practicable,as long as the desired transmission coverage is obtained.

End users access the wireless LAN through wireless LAN adapters.Wireless LAN adapters are implemented as PCMCIA cards in notebook orpalmtop computers, or as cards in desktop computers, and may beintegrated within hand-held computers. Wireless LAN adapters provide aninterface between the client network operating system (NOS) and thetransmission medium via an antenna. The nature of the wirelessconnection is transparent to the NOS.

Wireless LANs can range from simple to complex in both design andoperation. At its most basic, two PCs equipped with wireless adaptercards can establish an independent network whenever they are withinrange of one another. Such a network is generally referred to apeer-to-peer network. Ad hoc peer-to-peer “networks” require noadministration or preconfiguration. In this case, each client will haveaccess only to the resources of the other client, but not to a networkserver or host computer.

Installing an access point can extend the range of an ad hoc network,effectively doubling the range at which the devices can communicate.Because the access point is connected to the wired LAN, each client isafforded access to server resources as well as to other clients. Eachaccess point has the capacity to accommodate many clients, the specificnumber of clients depending on the number and nature of thetransmissions involved. Many applications exist in which a single accesspoint services from 15-50 client devices.

Access points have a finite range, on the order of 500 feet indoors and1000 feet outdoors. In a very large facility, such as a warehouse or acollege campus, more than one access point may be indicated. Accesspoint positioning is accomplished by means of a site survey. Theobjective is to blanket the intended coverage area with overlappingcoverage cells so that clients are free range throughout the areawithout losing network contact. The ability of clients to moveseamlessly among a cluster of access points is called roaming. Accesspoints often are designed to hand the client off from one to another ina way that is invisible to the client, thereby ensuring uninterruptedconnectivity.

Although, as may be gleamed from the above, wireless LANs offersignificant operational advantages, wireless LAN technology ischallenged by a number of phenomena. In particular, high-speed wirelessLAN transceivers, such as those intended to be compliant with the IEEE802.11(b) standard, require consistent strong signal in order tomaintain high data throughput. In an office environment, the RF signaltraveling between a client device (such as a notebook computer) and anaccess point (base station) will likely reflect off many objects andsurfaces, including walls, office furniture, and inhabitants, in routeto the receiving antenna. Due to the density of obstructions in anoffice environment, it is likely that a signal will reach the receivingantenna through multiple paths. Because of the resulting difference inpath length, disparate signals may arrive at the receiving antenna withrandomly variant phase relationships. That is, different versions of thesame signal will exhibit correspondingly different phase shifts intransmission between the access point and the portable client. This canresult in a phenomenon known as multipath fading, or multipathdistortion, which is primarily manifested as a time-varying signalamplitude at a receiver, in this instance, at a portable client. Infact, the IEEE 802.11(b) standard contemplates the adjustment oftransmitted data rate in response to variations in received signalstrength indication (RSSI), such as may result from multipath fading.

In an office environment with a stationary access point, there willexist areas of weak or null signal. These areas of signal nulls commonlyresult from multipath fading phenomena, in which randomly phased signalstravel different paths from the access point and effectively tend tocancel one another at the client. They are, accordingly, sometimescolloquially referred to as “fade bubbles”. Fade bubbles predictablyincrease in size with distance from the access point. If the transmitter(access point) and receiver (client computers) are both stationary, asin the situation, for example, where a notebook computer is placed on atable in a conference room and is communicating with a stationary accesspoint in the building, a condition may arise in which the signalstrength at the receiver is inadequate to enable transmission of data atthe specified maximum rate, or at all, until the user relocates thereceiver (computer) to a different location.

Even in situation where both the receiving device and transmittingdevice are stationary, the bubbles of signal minima drift unpredictablyif inhabitants or structures in the environment are continually orrandomly in motion. This effect can fortuitously, but randomly, improvethe throughput of a device that is sitting in a fade bubble as describedabove.

A method commonly employed as a response to multipath distortioninvolves the use of at least two antennas, with physical separationand/or RF isolation due to cross polarization. The antennas are thensaid to be spatially diverse, and the technique is commonly referred toas a diversity antenna system. The intended effect is that because thetransmitted signal emanates from two antennas, it will travel along twodifferent paths and, presumably, by subjected to different degrees ofmultipath fading. In this case, the receiver measures signal strengthfrom both antennas and selects the antenna when the RSSI orsignal-to-noise ratio (SNR) falls below a threshold. This method iseffective when the separation between antennas is sufficient so that atleast one antenna will likely be outside the fade bubble, therebyobviating the need to relocate the receiver. However, the implementationof diversity antennas in this manner increases the cost and complexityto the notebook computer design. Furthermore, the effectiveness ofdiversity antennas remains limited when the size of the fade bubblesbecome large with respect to the antenna separation.

Accordingly, what is desired is an approach to the mitigation ofmultipath fading phenomena in wireless LAN environments. A preferredapproach will tend to minimize the cost and complexity of the design,manufacture and operation of wireless LAN client devices, such asnotebook computers, PDAs and the like. In addition, it is desired thatthe approach be effective in circumstances where the client device isdeployed in a substantially fixed, or perhaps narrowly circumscribed,location, such as in a conference room.

In accordance with one aspect of the disclosure, an acceptable solutionis susceptible of implementation in a variety of ways. One approach isto use a single transmitting antenna at the access point and physicallytranslate or rotate the antenna continually. Typically access points inuse today have dual ½-wave dipole antennas connected through acustomer-accessible coaxial connector. Generally the access point willswitch the receiver between the two antennas to maximize received signalstrength. Generally, only one antenna will be used for transmitting.With this configuration, the transmitting antenna could be rotated 360degrees by a motor or could reciprocate continually, in the manner of awindshield wiper. An advantage of this scheme is that it may be madeavailable as a customer-installable option.

Although the above approach is viable and should result in goodperformance, it does require moving parts and therefore may result inthe generation of audible and electrical noise, and may also besusceptible to mechanical wear and tear. Another approach would be toattach an array of antennas to the access point and electrically switchbetween the antennas periodically. If the antennas have differentpolarization and radiation pattern characteristics, the desired effectof moving the fade bubbles can be accomplished with only a small spacingbetween the antennas. This suggests that the device could remain quitesmall.

SUMMARY

The above and other objects, advantages and capabilities are achieved inone aspect of the disclosure by a wireless communications system, suchas a wireless LAN, that comprises a plurality of client devices. In oneembodiment, the client devices are portable computers that include atransceiver for transmitting and receiving a wireless communicationsignal. The wireless communications system also comprises an accesspoint (alternatively referred to as a “hub”) that similarly incorporatesa transceiver that is coupled to an antenna array. An antenna controlleris coupled to the antenna array and operates to cause a substantiallycontinual variation in the radiation pattern of the antenna array duringperiods when the access point is transmitting. Continual variation ofthe radiation pattern of the transmitting access point antenna mitigatesthe effects of multipath fading by assuring that a stationary clientwill not be permanently positioned in a signal bubble area, that is, inan area of signal minima.

Another aspect of the disclosure is embodied in a method of mitigatingthe effects of multipath signal propagation in a wireless LAN. Themethod is characterized by substantially continually varying theradiation pattern of an antenna system in the course of the transmissionof a wireless signal from an access point to a client. The disclosurecomprehends numerous techniques for varying the antenna radiationpattern, including, but not limited to, varying the physical position ororientation of the antenna system, switching between antennas of two ormore different types, varying the length of one or more antennas in theantenna system, and varying the gain and/or phases of signals applied toantennas in the antenna system.

In another aspect of the disclosure, a computer system comprises atleast one portable computer, a network server, and an access point thatis coupled to the network server via a wired communications link andthat is coupled to the portable computer via a wireless communicationslink. An antennas system, including one or more antennas, is coupled tothe access point, and an antenna controller is coupled to the antennasystem. The antenna controller operates to cause the radiation patternof the antenna system to continually vary during the transmission of awireless signal from the access point to the portable computer, and, inso doing, mitigates multipath signal propagation effects.

In a further aspect, the disclosure may be exploited in a wireless LANas a method of mitigating the effects of multipath signal propagationbetween an access point and a receiving client. In accordance with themethod, a wireless signal is transmitted from the access point to thereceiving client and, contemporaneously, the radiation pattern of anantenna system is continually varied.

As an additional ramification of the disclosure, an antenna systemconstituent in a wireless LAN is operable to continually vary theradiation pattern imparted to a wireless signal that is caused topropagate from an access point to a client. The antenna system comprisesan antenna array coupled to the access point. An antenna controller iscoupled to the antenna array so as to continually vary the radiationpattern of the antenna so that even a substantially stationary client isassured to avoid permanent placement in a position of a signal null thatresults from multipath propagation.

In a still further embodiment, the disclosure is implemented in acomputer system that includes a client device coupled to a source ofnetworked resources. A communications device is coupled to the clientdevice via a wireless first communications link and is coupled to thesource of networked resources through a second, likely wired,communications link. An antenna system is coupled to the communicationsdevice in a manner that enables the communications device to transmitsignals to, and to receive signals from, the client device. The antennasystem is coupled to control means for varying the radiation pattern ofthe antenna system as signal is transmitted from the communicationsdevice to the client device.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject invention may be better understood, and its numerousobjects, features, and advantages made apparent to those skilled in theart, be reference to the following Drawings, in which:

FIG. 1 is a system block diagram of a typically configured wireless LANin which portable computer clients are coupled by a wirelesscommunications link to an access point. The access point is, in turn,coupled by a wired link to a network server or other networked resource.

FIG. 2 is an illustration of an access point that incorporates a spatialdiversity antenna system in which at least one antenna is manuallyrotatable about an axis that is orthogonal to the direction in which theantenna extends.

FIG. 3 is a block diagram of an access point in accordance with anembodiment of the invention. The access point is coupled to an antennaarray. The antenna array is coupled to antenna controller that causesthe radiation pattern of the array to vary continually.

FIGS. 4A, 4B, and 4C depict embodiments in which the radiation patternof the antenna array is varied by manipulation of the physical positionof at least one antenna in the array.

FIGS. 5A and 5B depict an embodiment of the invention in which theradiation pattern of the antenna array is varied by adjusting the lengthof at least one antenna in the array (FIG. 5A) or by selectivelyswitching between two or more antennas of different lengths or shapes.

FIG. 6 depicts an embodiment of the invention in which the radiationpattern of an antenna array is varied by varying the gain and/or phaseof signal(s) coupled to one or more antennas in the array.

The use of the same reference symbols in repeated instances, regardlesswhether in the same or in difference Drawings, is intended to indicatesimilar or identical elements.

DETAILED DESCRIPTION

For a thorough understanding of the subjection invention, reference ismade to the following Description, including the appended Claims, inconnection with the above-described Drawings.

Referring now to FIG. 1, as depicted therein, in one embodiment of theinvention there may be seen a computer system 10 comprising a pluralityof portable computers 11. The portable computers may be any one of anassortment of numerous types, including but not limited to notebookcomputers, laptop computers, personal digital assistants (PDAs), palmpilots, or the like. The portable computers are clients of and, in amanner to be described below, coupled to a network server 12. In amanner well understood in the art, the server is accessible on a LAN 13and operates to run administrative software that controls access to theLAN and to other resources available on the LAN, such as printers, diskdrives and the like (not shown). The server may also contain resourcesthat are made available to the client computers.

The computer system depicted in FIG. 1 also includes an access point 14that may be alternatively referred to as a “hub”. The access point iscoupled to the network server via a wired communications link 15 and iscoupled to the portable computer clients via a wireless communicationslink 16. That is, access point 14 operates to transmit to, and receivefrom, portable clients 11 a wireless signal by virtue of an associatedantenna 141. From another perspective, wireless communications link 16may be viewed as the aggregation of the separate links 16 a, 16 b, . . ., etc. that are established at the same or different times between theaccess point and each of the portable clients. In one embodiment of theinvention, the wireless communications link between and including theaccess point 14 and client computers 11 conforms to the current versionof IEEE 802.11b standard.

The access point may be one of the known and commercially availableproducts, such as, for example, the Cisco Aironet 340 Series AccessPoint. That product is characterized by the following specifications:

Data Rates Supported 1, 2, 5.5 and 11 Mbps Network Standard IEEE 802.11bFrequency Band 2400-2486.5 MHz Wireless Medium DSSS Media AccessProtocol CSMA/CA Network Operating Systems Microsoft Windows 2000, 98,95, NT and Supported CE Modulation DBPSK @ 1 Mbps DBPSK @ 2 Mbps CCK @5.5 and 11 Mbps Operating Channels 11 channels Simultaneous ChannelsThree

Other vendors of wireless LAN access points include Nortel Networks,Lucent Technologies, Sony Electronics, Inc., Intermec Technologies Corp,Cabletron Systems, MaxTech Corp., Symbol Technologies, BreezeCOM, andothers. Many, if not all, the above vendors also distribute PCMCIAwireless LAN circuit cards that are installed in the portable computersin order to enable the portable computers to participate in wirelesscommunications with the access point.

Many commercially available access points, such as the 340 Seriesalluded to above, exhibit dual antennas. In some products, both antennasare operative when the access point is receiving a signal, and only oneof the two is operative when the access point is transmitting. The dualantenna configuration is primarily intended to effect a diversityantenna system that counteracts the effect of multipath signalpropagation. That is, because the antennas are physically separate, thesignal path between the access point and a client differs in aggregatedistance, including reflections, between the two antennas. Thistechnique is referred to in the art as spatial diversity. Inevitably,one of the antennas will present a stronger, or otherwise preferable,signal. In an antenna system with spatial diversity, the signal at thepreferred antenna may be selected and the other signal ignored orsquelched. In fact, some access points include not only an antennasystem that exhibits spatial diversity, but also permit the antenna torotate about the point of connection with the access point. An exampleof such a configuration is seen in FIG. 2. As may be seen therein, anaccess point 20 includes a pair of spatially diverse (that is,physically separated) antennas 21 and 22. Each of the antennas iscoupled to the access point in a manner that allows manual rotation ofthe antennas about a virtual axis that extends horizontally, that is, ina direction orthogonal to an axis that extends the length of theantennas. In this manner, rotation of one or both of the antennas may beadjusted so as to adjust the radiation pattern of the antenna(s) andthereby optimize the signal received or transmitted by the access point.

In an additional, or alternative, effort to mitigate the deleteriouseffects of multipath signal propagation, antenna diversity may beincorporated into the client computer. Consonant with this approach,wireless LAN adapter card that is inserted into the portable client mayinclude dual antennas that similarly exhibit spatial diversity. In sucha system, two spatially diverse antennas are printed on the adaptercard. One antenna is selected for operation based on the RSSI or someother metric, for example SNR, that is deemed to quantify the quality ofthe communications link between the access point and the client. Ofcourse, this technique requires additional signal processing todetermine, at any point in time, which antenna is receiving thepreferred signal.

In accordance with the disclosure, an improved technique for mitigatingthe effect of multipath signal propagation is illustrated in FIG. 3.FIG. 3 depicts an access point 31 that is coupled to an antenna array35. Typically, the access point will be coupled to antenna array 35through a connector 32, a coaxial cable 33, and a connector 36 at theantenna array. The antenna array may include one or more individualantennas. Three are shown in FIG. 3. Operation of the antenna array iscontrolled by an antenna controller 34 coupled to the antenna array, asdepicted in FIG. 3. Numerous forms of antenna arrays are contemplated bythe disclosure. The design and operation of the antenna controller willconsequently vary, in accordance with techniques understood by thoseskilled in the art. Specifically, antenna controller and antenna arraycooperate so that the radiation pattern of the antenna array is causedto continually vary. In this manner, the characteristics of the signalat the location of the client portable computer are likewise continuallyvarying. In one embodiment, a T_(x) signal that is available at accesspoint 32 indicates that the access point is in a transmit-data mode. TheT_(x) signal may be coupled to the antenna controller to enableoperation of the antenna controller only when the access point istransmitting.

A number of exemplary approaches to the implementation of an antennasystem are depicted in FIGS. 4, 5, and 6. It is to be understood,however, that the techniques illustrated in those FIGUREs are notexhaustive. Practitioners will understand that the invention extends toany implementation in which the radiation pattern of an antenna systemis caused to continually vary, thereby ameliorating the effects ofmultipath propagation and attendant signal fading phenomena.

FIG. 4 depicts an implementation in which the radiation pattern of anantenna system is caused to vary as result of the variation of thephysical position and/or orientation of an antenna system, or a singleantenna.

In the embodiment of FIG. 4A, the antenna system consists essentially ofa single antenna 43, which may be a dipole antenna. The dipole antennais coupled to the access point through a connector and cable arrangement(not shown). An antenna controller, in the form of a motor 41, ismechanically coupled, through a coupling mechanism illustrated aselements 42 a and 42 b, to the shaft of a dipole antenna 43. The motoris actuated when the access point is operating in the transmit mode and,when so actuated, causes the antenna to rotate about an axis defined bythe antenna shaft. Of course, as the antenna rotates about the verticalaxis defined by the antenna shaft, the radiation pattern of the antennacontinually varies. Similarly, the quality of the signal received by theclient computer will likewise vary in response to variation in theantenna radiation pattern.

It is worthy of note that control of the antenna radiation pattern asdescribed above is open-loop in nature. That is, the antenna is causedto rotate independent of any information derived from the quality ofsignal actually present at the client. Consequently, the signal at theclient input can be expected to continually vary between a relativesignal minima, in which the client input signal is relatively leasteffective, and a relative maxima, in which the client input signal isrelative most effective. SNR, BER or packet error rate will be directlyrelated. The end result is that the distant client (on the fringe of acoverage area) will achieve data throughout that is a fraction of thenetwork's optimal capability. However, for data applications thissituation may be relatively transparent to the user, and is muchpreferred to a situation in which data transmission is entirely impeded.The described operation is in contrast to the operation of knowndiversity antenna systems or antenna steering systems, the operation ofwhich is generally predicated on the selection of a “best” signal fromone or more available signals, or on the optimization of the radiatedsignal in response to feedback derived from the received signal. Thosetechniques necessarily requiring a signal processing, and associatedcircuit complexity, that is not required by the subject disclosure.However, by continually varying the radiation pattern of the antenna,the subject disclosure assures that a client device avoids a situationin which the device is, for an extended duration, fixed in a location,such as a conference room, that corresponds to a signal null, or“bubble”, as referred to above.

FIG. 4B illustrates another approach to the variation of an antennaradiation pattern through manipulation of the physical position and/ororientation of an antenna system. In the embodiment of FIG. 4B, theantenna position is again driven by a motor 41, but in this instance theantenna travels a substantially linear path between the extremities of alinear track on which the antenna is supported.

In FIG. 4C, the antenna is shown mounted on a carousel, and as thecarousel rotates about its axis, the antenna is caused to travel asubstantially circular path. Again, the radiation pattern of the antennavaries in response to the instantaneous position of the antenna.

FIG. 5 is directed to a technique in which the radiation pattern of theantenna system is continually varied during an access point transmissionby varying the length of an operative antenna in the antenna system. InFIG. 5A, the length of an antenna is varied by sequentially adding anincremental length, or lengths, of antenna to a fixed length. Theantenna controller in this embodiment comprises a switch 51 thatselectively and sequentially connects incremental antenna length(s), 52b, 52 c, . . . , 52 n, to the fixed length 52 a. The antenna controllerswitch may take the form of an electromechanical, mechanical, orelectronic switch. Electronic switches at RF may be realized through theuse of PIN diodes or microelectromechanical systems.

FIG. 5B illustrates an embodiment according to which the antenna lengthis varied through the operation of an antenna controller thatselectively switches between two or more antennas of different lengths.

FIG. 6 illustrates an embodiment in which the radiation pattern of anantenna array is varied by varying the gain and/or phase applied to asignal that is then coupled to individual antennas in an antenna array.In this embodiment, the antenna controller is implemented as again/phase controller that adjusts the gain and phase of a signal andcouples disparate gain- and phase-adjusted signals to individualantennas in an antenna array. For a discussion of applicable designapproaches, known to those skilled in the art of antenna system design,see Garrett T. Okamoto, Smart Antenna Systems and Wireless LANs, KluwerAcademic Publishers (1998), hereby incorporated by reference in itsentirety.

With specific reference now to the embodiment exemplified in FIG. 6, itmay be seen that the transmittal signal output, T_(x), at access point31 is coupled through connector 32 and coaxial cable 33 to a gain/phasecontroller 60 that may include a signal splitter 61, a number ofgain/phase modules 621, 622, and 623, and a gain/phase controller 63.The output of gain/phase controller 60, at the outputs of gain/phasecontroller 60, at the outputs of gain/phase modules 621, 622, and 623,is coupled to an antenna array 35.

Specifically, the T_(x) output of access point 31 is coupled to an inputof signal splitter 61. Signal splitter 61 provides at its out a numberof replicates of signal T_(x). For strictly pedagogical purposes, signalsplitter 61 is shown to provide three output, but the inventioncontemplates any number greater than one. Signal splitter 61 may includeactive devices, but may be constructed from discrete or dumped passiveelements. It is likely preferred, but not required, that the outputs ofsignal splitter 61 be maintained substantially identical in amplitudeand phase.

The outputs of signal splitter 61 are coupled to respective inputs ofgain/phase (gain/Φ) modules 621, 622, and 623. Again, three gain/Φmodules are illustrated in FIG. 6, but the invention is not to beunderstood as restricted to a number certain of gain/Φ modules.

In accordance with any appropriate one of numerous known techniques,gain/Φ controller 63 operates to continually vary the gain and/or phaseapplied by the gain/Φ elements to the respective outputs of signalsplitter 61. As a result, the T_(v) output at the access point isdecomposed into, for example, three separate signals. In a mannerdetermined by gain/Φ controller 63, gain and or phase adjustments aremade to the separate signals. Gain/Φ controller 63 is designed, eitherthrough operation of constituent hardware or under software programcontrol, or a combination of both, to continually vary the gain and/orphase adjustments applied to each of the signals. Embodimentscontemplate that each of the separate signals experience either or bothgain and phase adjustments, at the discretion of the system designer oras required by externalities. The salient result being that, whateverthe precise nature of the adjustments made to the separate outputs ofsignal splitter 61, the output of gain/phase control system 6, which isthe composite of the signals at gain/Φ modules 621, 622, and 623, isapplied to individual antenna(s) (not shown) in antenna array 35 in amanner that results continual variation in the radiation pattern that isassociated with the antenna array.

Accordingly, from the above, it may be seen that the disclosure, in itsnumerous embodiments, affords is straightforward approach to the problemof multipath signal fading in the context of wireless LAN systems. Ingeneral, the solution includes continually varying the radiation patternof an antenna system that is associated with the access point.

Of course, physically moving antenna elements in the antenna system, orotherwise modifying the antenna system radiation pattern continually,does not result in operation in which the fade bubbles will necessarilybe eliminated. However, it is insured that the bubbles will always bemoving, thereby virtually ensuring that a stationary computer within therated system range of the system will avoid complete loss of servicebecause of multipath fading.

In another approach to implementation of at least one aspect, an antennaarray may comprise a number of distinct antennas, each of which exhibitsa characteristic radiation pattern that respectively differs from theradiation patterns exhibited by other antennas. This may be accomplishedby providing antennas of different types or configurations, such as, butnot limited to a loop antenna, a dipole antenna, a folded dipoleantenna, a whip antenna, a log periodic antenna, and the like. Theantenna controller is then interposed between the access point and theantenna array and operates to selectively sequentially connect thesignal to be transmitted at the access point to one antenna in thearray. Subsequently, the signal is decoupled from the first antenna andthen coupled seriatim to each of the other antennas, so that theeffective radiation pattern of the array continually varies, at leastduring the transmit cycle of the access point.

From the above Description, it may be readily apprehended that thesalient characteristic of the disclosure is the mitigation of theconsequences of multipath fading by varying the radiation pattern of anantenna system associated with an access point, or equivalent hub, in awireless data transmission system. In one embodiment, the variation isdesigned to be substantially continual, at least during transmittingsession engaged in by the access point. However, in the context of theabove Description, continual variation of the radiation pattem is tomean only that the radiation pattern of the antenna system does notremain constant, that is, varies, through at least a substantial portionof the transmitting session. In fact, the variations may be discrete ordiscontinuous, as when operation is switched between two antennas withdistinctly different radiation patterns. In this regard, it should beunderstood that the radiation pattern will be deemed to vary continuallyif, for example, the duty cycle of each antenna is substantially lessthan the duration of a client session and at least one antenna isoperating at any time during the session.

Accordingly, although an exemplary embodiment has been described indetail herein, those possessed with ordinary skill in the art willreadily apprehend various changes and modifications in form and detailto the subject matter so described, without departure from the spiritand scope of the disclosure. Consequently, the scope of the disclosureis not properly delimited by the above Description, but is to beestablished with reference to the appended Claims, and equivalentsthereto.

What is claimed is:
 1. A computer system comprising: a portablecomputer; a network server; an access point coupled to the networkserver via a wired communications link and coupled to the portablecomputer via a wireless communications link; an antenna system coupledto the access point; and an antenna controller coupled to the antennasystem for continually varying the radiation pattern of the antennasystem while a wireless signal is transmitted from the access point tothe portable computer, the antenna controller comprising: a signalsplitter having an input coupled to a signal provided by the accesspoint for transmission to the client and having a plurality of outputs;a plurality of gain/phase modules, each of the gain/phase modules havinga signal input coupled to an output of the signal splitter and having anoutput; and a gain/phase controller having a plurality of outputs, eachrespective coupled to a control input of one of the gain/phase modules,the gain/phase controller for adjusting the gain and/or phase impartedto the signals applied to the signal inputs of the gain/phase modules.2. A computer system as defined in claim 1, wherein the antennacontroller varies the radiation pattern of the antenna system by varyingthe physical position of an antenna in the antenna system.
 3. A computersystem as defined in claim 1, wherein the antenna controller varies theradiation pattern of the antenna system by varying the physicalorientation of an antenna in the antenna system.
 4. A computer system asdefined in claim 3, wherein the antenna controller rotates an antenna inthe antenna system about an axis of the antenna.
 5. A computer system asdefined in claim 1, wherein the antenna controller varies the length ofan antenna in the antenna system.
 6. A computer system as defined inclaim 1, wherein the antenna controller varies the gain and/or phase ofa signal to be applied to at least one antenna included in the antennasystem.
 7. A computer system comprising: a client device; acommunications device coupled to the client device via a wireless firstcommunications link and coupled to a source of networked resources via asecond communications link; an antenna system coupled to thecommunications device in a manner that enables the communications deviceto transmit signals to and to receive signals from the client device;and control means comprising an antenna controller coupled to theantenna system for varying the radiation pattern of the antenna systemas information is transmitted from the communications device to theclient device, the antenna controller comprising: a signal splitterhaving an input coupled to a signal provided by the communicationsdevice for transmission to the client and having a plurality of outputs;a plurality of gain/phase modules, each of the gain/phase modules havinga signal input coupled to an output of the signal splitter and having anoutput; and a gain/phase controller having a plurality of outputs, eachrespective coupled to a control input of one of the gain/phase modules,the gain/phase controller for adjusting the gain and/or phase impartedto the signals applied to the signal inputs of the gain/phase modules.8. A computer system as defined in claim 7, wherein the control meansoperates to vary the radiation pattern of the antenna system bycontinually varying the spatial orientation of at least an antenna inthe antenna system.
 9. A computer system as defined in claim 7, whereinthe control means operates to vary the radiation pattern of the antennasystem by sequentially selecting one or more antennas for operation froman array of antennas that individually exhibit mutually differentradiation patterns.
 10. A computer system as defined in claim 7, whereinthe control means operates to vary the radiation pattern of the antennasystem by continually varying the gain and/or phase of a signal that isapplied to at least one antenna in the antenna system for transmissionto the client device.
 11. A computer system as defined in claim 7,wherein the antenna controllers is coupled to the communications deviceto receive a T_(x) signal that indicates that the communications deviceis engaged in a transmitting session.
 12. A computer system as definedin claim 11, wherein the antenna system consists essentially of a singleantenna and wherein the antenna controller operates to cause theradiation pattern of the antenna to vary continually during atransmitting session by continually varying the spatial orientation ofthe antenna.
 13. A computer system as defined in claim 12, wherein theantenna controller effects open-loop control of the radiation-pattern ofthe antenna system.
 14. A computer system as defined in claim 7, whereinthe antenna system comprises a plurality of antennas, each of which ischaracterized by a respective radiation pattern, and wherein the antennacontroller is coupled to the communications device to receive a T_(x)signal that indicates that the communications device is operating in atransmit mode.
 15. A computer system comprising: a client device; acommunications device coupled to the client device via a wireless firstcommunications link and coupled to a source of networked resources via asecond communications link; an antenna system coupled to thecommunications device in a manner that enables the communications deviceto transmit signals to and to receive signals from the client device;control means coupled to the antenna system for varying the radiationpattern of the antenna system as information is transmitted from thecommunications device to the client device; the antenna systemcomprising a plurality of antennas, each of which is characterized by arespective radiation pattern; the control means comprising an antennacontroller that is coupled to the communications device to receive aT_(x) signal that indicates that the communications device is operatingin a transmit mode, the antenna controller comprising: a signal splitterhaving an input coupled to a signal provided by the communicationsdevice for transmission to the client and having a plurality of outputs;a plurality of gain/phase modules, each of the gain/phase modules havinga signal input coupled to an output of the signal splitter and having anoutput; and a gain/phase controller having a plurality of outputs, eachrespective coupled to a control input of one of the gain/phase modules,the gain/phase controller for adjusting the gain and/or phase impartedto the signals applied to the signal inputs of the gain/phase modules.